The present invention relates to a method of coating a film on an object such as a semiconductor wafer or an LCD substrate, an apparatus for coating a film on such an object, and an apparatus for counting particles existing in the coating liquid.
In the manufacture of a semiconductor device, a circuit pattern is formed by means of so-called photolithography. The photolithography comprises the steps of: coating a photoresist on a semiconductor wafer, exposing the photoresist to light by using a photomask, and developed the photoresist thus exposed to light.
In the photoresist-coating step, the resist liquid is applied on to the center part of the semiconductor wafer from a nozzle located above the wafer, while the wafer held on a spin chuck is spinning at high speed. The resist liquid thus applied spread by virtue of the centrifugal force the wafer exerts while spinning. As a result, a resist film having an uniform thickness is formed on the entire surface of the semiconductor wafer.
The semiconductor wafer with the resist film coated on it is subjected to heat treatment, light-exposure, development and etching. A circuit pattern is thereby formed on the semiconductor wafer. The circuit pattern may not be a desired one if the resist film contains particles.
To form a resist film containing as few particle as possible, a filter is interposed between the nozzle and the resist liquid source to filter out particles from the resist liquid. The efficiency of the filter gradually decrease with time. Hence, the filter may fail to filter out particles after a long use. Unless the filter is replaced with a new one, many particles will remain in the resist liquid.
To decide whether or not the filter should be replaced with a new one, it is necessary to determine how much the efficiency of the filter has decreased. To this end it is required that the particles in the resist liquid be counted before the liquid is applied to semiconductor wafers. It is proposed that the particle counters commercially available be used to count the particles in the resist liquid.
Here arises a problem. The conventional particle counters are designed to count particles existing in low-viscosity liquids such as pure water and hydrofluoric acid, not to count particles in a high-viscosity liquid such as resist liquid which has viscosity of several cP to several hundred cP. If a conventional particle counter is placed between the nozzle and the semiconductor wafer and used for a long time to count particles in the resist liquid applied from the nozzle, the resist liquid sticks to the inner wall of the optical cell of the counter. Much time and labor are required to wash the particle counter. In view of this, the conventional particle counter cannot be used in an in-line fashion as is employed to count particles existing in pure water or hydrofluoric acid.
The counter must therefore be located outside the line of manufacturing semiconductor devices. In this case, the resist liquid must be sampled, and samples must be supplied to the particle counter. This also requires much time and labor.
The conventional particle counter cannot be used to count particles in the resist liquid, for another reason. It applies a light beam, such as a laser beam, to a liquid to count particles existing in the liquid. When the conventional particle counter applies a light beam to the resist liquid, the resist liquid emits light. This makes it difficult for the counter to count particles in the resist liquid with a sufficiently high accuracy.